1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Base stations in wireless communication systems provide wireless connectivity to users within the geographic area, e.g., a cell or sector, associated with the base station. The wireless communication links between the base station and each of the users typically include one or more downlink (or forward) channels for transmitting information from the base station to the mobile unit and one or more uplink (or reverse) channels for transmitting information from the mobile unit to the base station. Transmissions over the uplink and/or downlink channels may interfere with each other. For example, in wireless communication systems that implement asynchronous code division multiple access (CDMA), a mobile unit in a first sector or cell experiences interference from other mobile units in the first sector or cell, as well as interference from mobile units in other sectors or cells. Time division multiple access (TDMA) or orthogonal frequency division multiple access (OFDMA) can reduce or eliminate intra-sector interference, but inter-sector interference still remains.
The spectral efficiency achievable on the uplink of conventional cellular networks is fundamentally limited by co-channel interference between mobile units sharing the same channel. Consequently, intra-sector interference and/or inter-sector interference begins to diminish the return in spectral efficiency that results from allocating additional transmitter power to mobile units when this limit is approached and the signal-to-interference-plus-noise ratio (SINR) for each mobile unit begins to saturate. The spectral efficiency of the wireless communication system can approach the limits set by co-channel interference when sophisticated error correcting codes, adaptive modulation techniques, and/or incremental redundancy techniques are implemented. However, they cannot overcome this limit. The SINR pattern in the wireless communication system can be improved by imposing a frequency reuse pattern on the network. However, the resulting spectral efficiency is typically even lower than in systems that do not include frequency reuse, at least in part because of the incurred loss in bandwidth within each sector.